This disclosure relates to microwave and millimeter wave circuits and particularly to transitions for coupling signals between microstrip and waveguide transmission lines.
Microwave and millimeter wave circuits may use a combination of rectangular and/or circular waveguides and planar transmission lines such as stripline, microstrip, and co-planar waveguides. Waveguides are commonly used, for example, in antenna feed networks. Microwave circuit modules typically use microstrip transmission lines to interconnect microwave integrated circuit and semiconductor devices mounted on planar substrates. Transition devices are used to couple signals between microstrip transmission lines and waveguides.
Compact, highly-integrated radio frequency (RF) assemblies include, among other things, a power amplifier, a wirebond transition to a circuit board microstrip conductor, a second transition to a radiating element (such as a probe or printed antenna), and a thermal control substrate (such as a heat spreader). The components convey RF energy from the power amplifier (PA) to the radiating element. In turn, the radiating element may couple the RF energy to an output waveguide. The waste heat from the components (especially the PA) is controlled and redirected by the heat spreader in order to prevent degradation and/or premature failure of the electronics.
Traditional methods of employing heat spreaders in such assemblies often use individual heat spreaders under each microwave integrated circuit, chip, or other electronics in the assembly, a wirebond transition to microstrip, and then another transition to a radiating element. These transitions are somewhat fragile and prone to de-tuning from mechanical shocks. They are also labor-intensive to fabricate correctly and thus costly. Furthermore, such transitions can be very frequency-sensitive, thus limiting the utility of a particular transition design to a narrow range of either center frequency or bandwidth. In particular, standard transition techniques used at low frequency do not work well in high frequency applications because the transition has more loss and less bandwidth due to the tuned length of the microstrip transition in the circuit board.
Other transition methods known in the related arts include circuit E-probe, post E-Probe, and patch antenna transitions. Some prior art patch antenna transitions are described below with reference to FIGS. 1 and 2.
A prior art circuit E-probe transition is a fully micro-machined, finite ground, coplanar line-to-waveguide transition. The E-probe injects the transmit signal into a micro-machined slot, resulting in an E-field. The E-field then propagates into the waveguide. Such circuit E-probe transitions are described in, for example, Yongshik Lee, et al., Fully Micromachined Finite-Ground Coplanar Line-to-Waveguide Transitions for W-Band Applications, IEEE Trans. on Microwave Theory and Techniques, Vol. 52, No. 3, March 2004, p. 1001-1007.
In a prior art post E-probe transition to a rectangular waveguide, a co-planar waveguide (CPW) port is coupled to a post, which is located within a cavity formed on a quartz substrate. The cavity is typically formed of multiple, stacked layers of silicon. Electromagnetic energy injected at the CPW port causes the formation of an E-field in the cavity, which then couples through the waveguide port and thence down the waveguide (not shown). Such Post E-probe transitions are described in, for example, Yuan Li, et al., A Fully Micromachined W-Band Coplanar Waveguide to Rectangular Waveguide Transition, Proc. of IEEE/MTT-S International Microwave Symposium, 3-8 Jun. 2007, p. 1031-1034. Another implementation of a post E-probe transition is described in Nahid Vahabisani, et al., A New Wafer-level CPW to Waveguide Transition for Millimeter-wave Applications, 2011 IEEE International Symposium on Antennas and Propagation (APSURSI), 3-8 Jul. 2011, p. 869-872.
FIG. 1 depicts a prior art, fully micro-machined, W-band waveguide-to-grounded coplanar waveguide transition for 91-113 GHz applications 300. This transition utilizes via holes 310 to couple energy from port 320 to waveguide 330. Such transitions are typically used with patch antennas. This design is further described in Soheil Radiom, et al., A Fully Micromachined W-band Waveguide-to-Grounded Coplanar Waveguide Transition for 91-113 GHz applications, Proc. of the 40th European Microwave Conference, 28-30 Sep. 2010, p. 668-670.
FIG. 2 depicts another prior art transition used in patch antennas. This prior art transition 400 does not use via holes, but instead employs a microstrip 405, probe 410, and a patch element 420 (with surrounding ground plane 425) to couple energy into waveguide 430. Patch element 420 is formed on substrate 440. This design is further described in Kazuyuki Seo, et al., Via-Hole-Less Planar Microstrip-to-Waveguide Transition in Millimeter-Wave Band, 2011 China-Japan Joint Microwave Conference Proceedings (CJMW), 20-22 Apr. 2011, pp. 1-4.
Raytheon Company has previously designed a similar printed antenna transition addressing some of the same issues, as illustrated in FIG. 3. Printed circuit antenna 510 is provided on substrate 520 and connected to a transmitter (such as a power amplifier, not shown) located on pad 530 by a printed circuit trace 540. Energy is coupled to a waveguide (not shown) by means of via holes 550 in substrate 520. Antenna 510 is a quarter-circle or half-Vivaldi antenna, itself well-known in the art. This design is further described in U.S. Published Applications US2011/0102284 and US2010/0210225, incorporated herein by reference in their entireties.
In order to reduce losses, it is therefore desirable to minimize the use of transitions in coupling the energy from the PA to the waveguide, while at the same time providing a coupling scheme capable of operation and scalability over a wide range of operating center frequencies and bandwidths.